1. Field of the Invention
The present invention relates to an improved electrophotographic apparatus, and particularly to an electrophotographic apparatus utilizing a photosensitive member essentially composed of an electroconductive layer, a photoconductive layer and an insulating layer, and capable of providing a satisfactory electrostatic latent image achieving a desired contrast level and still avoiding background fog at the image development.
2. Description of the Prior Art
In the prior art there are already known various apparatus capable, based on electrophotographic processes, of forming an image on a photosensitive member provided with a photoconductive element.
Particularly an electrostatic latent image of an elevated contrast is obtainable with an apparatus utilizing a photosensitive member essentially composed of an electroconductive layer, a photoconductive layer and an insulating layer for forming the electrostatic latent image through the steps of a primary electrostatic charging, an imagewise exposure, a simultaneous electrostatic charging of inverted polarity or AC static elimination, and if necessary a whole surface exposure according for example to the electrophotographic processes disclosed in the U.S. Pat. No. 3,666,363 or 3,734,609, and such apparatus have been identified as extremely useful for practical purposes.
In the prior electrophotographic apparatus, the use of magnetic brush developing device has been quite common for the development of electrostatic latent image formed on the photosensitive member because of compact structure and easy maintenance of such device. However, the indiscriminative contact of the toner particles with both the image or dark area and the non-image or light area, as observed in the above-mentioned magnetic brush development, tends to result in a development of non-image area, or, so-called toner fog.
For example in the magnetic brush development, the potential in the non-image area is preferably maintained within a range from -50 to -100 V since a more positive potential will result in the above-mentioned toner fog while a more negative potential will also lead to an another drawback of carrier deposition. In any development method, therefore, it is essential, for achieving a satisfactory image quality, to maintain a suitable potential in the non-image or background area on the photosensitive member which generally corresponds to the light or background area of the original image.
Also in practical electrophotographic apparatus, the field maintenance is often carried out in such a manner to elevate the potential of electrostatic charging in order to compensate the reduced dark resistance resulting from certain deterioration of the photosensitive member after repeated use thereof or the deficient concentration of developer thereby sustaining the image quality.
In case of an apparatus based on the electrophotographic process as mentioned in the foregoing, such maintenance operation is conducted by modifying the potential of the primary electrostatic charging.
This is based on a fact that the surface potential V of the photosensitive member provided with an insulating layer is correlated with the amount of discharge Q resulting from thus modified primary charging by the following equation: EQU Amount of discharged electrostatic charge Q.varies.Surface potential V
It will be noted, however, that such increase in the amount of discharge in the primary charging will not only elevate the potential of dark area on the photosensitive member corresponding to the dark area of the original light image but also result in an increased potential in the background area thereof, and thus obtained electrostatic latent image with increased background potential tends to show fog formation at the development thereof.
In order to eliminate such fog formation it is necessary to obtain an appropriate background potential by regulating the intensity of the secondary AC corona static elimination. Thus the adjustment of the primary charging potential also necessitates the adjustment of intensity of the AC corona static elimination. Although the surface potential control by the AC corona discharge has been achieved by adding a bias voltage to a sinusoidal AC voltage, such method generally requires an elevated voltage in order to maintain the background potential at -50 to -100 V in an electrophotographic apparatus and is inevitably dangerous particularly when the apparatus is made smaller. Such method of control by adding a bias voltage to a sinusoidal AC voltage is specifically disclosed for example in the U.S. Pat. No. 3,714,531, particularly in FIG. 4 and column 3, lines 43 to 51 thereof.
Also as an another means of control there is proposed a method of causing a distortion in the sinusoidal AC output current. Such distortion can be achieved by connecting a variable impedance in parallel with the AC corona static eliminator to the output of a power supply and thereby suitably controlling the positive and negative sides of the corona discharge current, but the voltage-current characteristics of the transformer inevitably generate a voltage as high as several kilovolts applied to the regulating variable impedance. Due to the presence of such high voltage, this method can also be similarly dangerous as the foregoing one unless suitable measures are taken for safety. The specific structure of such regulating element between the discharge wire and the high-voltage power supply is disclosed for example in the specification of the above-cited patent, column 3, lines 9 to 34 and FIG. 3 thereof.
However the means based on these methods, even if incorporated in the conventional electrophotographic apparatus, often involves difficult adjustments and usually allows to obtain optimum background potential only after prolonged adjustment procedure.
The present invention has been reached in consideration of the above-explained state of the art.